THERMOMETRY AND HYDROGEOCHEMISTRY OF THE SOUTHERN BORDER OF THE SOUTH PYRENEAN FORELAND BASIN

Abstract

The main objective of the study was a thermal and hydrochemical characterisation of the Osona depression located south of the South Pyrenean foreland basin. Thermal gradient was measured in 36 water wells. Thermal conductivity was also measured for the most significant lithologies of the area. Thermal gradient data showed a high scatter (from 6 mK/m to 103 mK/m) indicating a complex groundwater regime related to basement structure, faulting and multilayered configuration of the aquifer. No relationship between low thermal gradients and recharge zones was found. 107 water points were sampled and chemically analysed. 30 water samples were isotopically studied. These hydrochemical and isotopical analyses did not reveal geothermal processes. This was interpreted as being due to mixing of geothermal fluids and shallower waters. Geothermometric techniques to obtain reservoir temperatures failed. The regional groundwater pattern corresponds to diverse and independent hydraulic systems. Deep groundwater flow through the lower permeable layers is responsible for positive thermal anomalies. Heat diffusion in marly units at depth and the presence of faults are most probably the factors controlling the geothermal field.